The invention pertains to the field of sigma-delta modulators. Conventionally, a sigma-delta modulator comprises an amplification unit, an analog-to-digital converter and a digital-to-analog converter. The modulator renders the analog-to-digital conversion by the analog-to-digital converter, contained in the modulator, slower but more precise, by reducing the quantification noise of the analog-to-digital converter. The modulator can be subdivided up into a direct chain, comprising the amplification unit and the analog-to-digital converter, and a feedback loop comprising the digital-to-analog converter.
An efficient sigma-delta modulator has high value gain. Indeed, the quantification noise may be reduced by a factor substantially equal to this gain. However, the value of the gain is limited by constraints related to the stability of the modulator. The basic condition of stability of the system having a feedback loop may be stated as follows: the phase of the system must be higher than xe2x88x92180 degrees when the gain of the system as a function of the frequency is equal to zero decibel. Now, a delay in propagation of the signal within the system has the effect of reducing its phase, and therefore of lowering its stability. Consequently, a delay in propagation between the input of the analog-to-digital converter and the output of the digital-to-analog converter has the effect of lowering this stability, and even leading to instability in the modulator. One of the objects of the invention is to propose a sigma-delta modulator used to compensate for the effects, on the stability of the modulator, of the delay in propagation between the input of the analog-to-digital converter and the output of the digital-to-analog converter. This sigma-delta modulator is used to compensate for the delay of the analog-to-digital converter and the delay of the digital-to-analog converter.
For this purpose, a first prior art approach consists in reducing the gain of the modulator so that the modulator is stable despite the delay in propagation. However, this reduction in gain causes a reduction in the performance characteristics of the modulator, especially in terms of reduction of the quantification noise.
A second prior art approach consists in reducing the internal sampling frequency of the modulator in order to mask the propagation delay. The internal sampling frequency of the modulator will often be the sampling frequency of the converters. Since the over-sampling rate is the ratio between the output frequency of the information from the modulator and the internal sampling frequency of the modulator, at a given output frequency the reduction in the internal sampling frequency corresponds to a reduction in the over-sampling rate. Now, the reduction in the over-sampling rate is also expressed by a reduction in gain with the same drawbacks as for the first prior art approach.
The invention proposes a modulator comprising a compensation filter used to compensate for the delay in propagation between the input of the analog-to-digital converter and the output of the digital-to-analog converter. This compensation filter is used to preserve the impulse response that the modulator would have had in the absence of a propagation delay. Since there is a one-to-one correspondence between the impulse response of the modulator and the gain of the modulator as a function of frequency, this compensation filter is also used to preserve the gain as a function of the frequency that the modulator would have had in the absence of a propagation delay. To this end, the compensation filter reconstitutes the beginning of this impulse response that is not available because of the delay in- propagation.
According to the invention, there is provided a sigma-delta modulator having an input terminal and an output terminal, comprising:
a direct chain connecting the input terminal of the modulator to the output terminal of the modulator and successively comprising an amplification unit comprising an amplification function with gain greater than unity on a predetermined frequency domain, the input terminal of the modulator being connected to a first direct input of the amplification unit, and an analog-to-digital converter;
a feedback loop connecting the output of the analog-to-digital converter to a second inverter input of the amplification unit and comprising a digital-to-analog converter;
and having a propagation delay xcex94t between the input of the analog-to-digital converter and the output of the digital-to-analog converter;
characterized in that the sigma-delta modulator also comprises:
a subtractor located in the direct chain between the amplification unit and the analog-to-digital converter, the output of the amplification unit being connected to a first direct input of the subtractor, an output of the subtractor being connected to the input of the analog-to-digital converter;
a compensation filter located between the output of the subtractor and a second inverter input of the subtractor;
and in that, when considering an impulse response of the modulator, at the output of the subtractor, to an impulse sent at the output of the subtractor, comprising a first part covering a first time interval [0; T [ with Txe2x89xa7xcex94t and a second part covering a second time interval [T; ∞[, the compensation filter is designed to contribute to the first part and the amplification unit is designed to contribute only to the second part.